It is well known that polymeric materials which are thermoplastic and usually soluble in many organic solvents may be transformed by chemical reaction with suitable reagents into products of more complex chemical structure and higher molecular weight so they become non-thermoplastic, highly resistive to solvation by ordinary solvents or assume other new characteristics as a result of the chemical reaction. New polymeric products formed by such processes may acquire elastomeric qualities and their increased resistance to solution or attack by solvents is characterized by formation of gels when brought in contact with numerous solvents, this gelation being considered to constitute a measure of the extent of reaction of the initial polymeric material with the modifying agent.
Probably the most historic example of chemical reactions of this type involving polymer materials is the vulcanization of rubber with sulfur or sulfur producing reagents. In any event, this broad type of chemical reaction is referred to in the chemical arts by various terms dependent, at least to some extent, upon the degree or severity of reaction of the polymer material with the modifying agent. The terms "vulcanizing", "curing", "cross-linking" and "chain-extending" have been applied to this general form of operation and the modifying agents employed in such procedures are variously referred to as "curing agents", "vulcanizing agents", "cross-linking agents", "chain-extending agents" and other terms. For the purpose of the description of the present invention, the terms "curing" and "curing agent" will be employed for reference generally to this type of chemical reaction. In this context, the invention concerns curing of a specific class of polymeric materials which are known in the polymer art as carboxyl containing polymers. The curing agents are the new multiaziridines.
Where the "curing" is in the nature of chain extension, it is believed this takes place as a polyaddition with the formation of secondary amino groups. Where the "curing" is preliminarily in the nature of cross-linking, one explanation of the results of this invention is that secondary amino groups are known to react with ethylenimine rings much easier than with the propylenimine or butylenimine ring, forming, with ring-opening, substituted ethylene diamines. Secondary ammonium groups can be formed on the polymer chain by reaction of the --NH-- groups with protons from the carboxyl groups present. Further reaction with ethylenimine di-adduct would then yield cross-linked polymer chains with the formation of ethylene diamine units.
Another possibility to account for the cross-linking reaction is a trimerization or oligomerization of the diaziridino-compound under the influence of protons from the dicarboxylic acid present. The resulting aziridine-trimers or oligomers subsequently can react with the dicarboxylic acid to form the cross-linked polymer.
Compounds which have been found useful as curing agents for polymer materials generally contain in their molecular structure a plurality of groups which can react with the molecules of the polymeric material in the curing, i.e., so-called "functional groups". The transformation of the polymer material into a more temperature and solvent resistant form is explained on the basis that polymer molecules are joined together by reaction with these functional groups into a three-dimensional network or, at the very least, a product of much greater molecular weight and complex structure. Accordingly, virtually all classes of compounds which possess a plurality of reactive or functional groups have been investigated as possible curing agents for a wide variety of polymer materials.
The aziridino group possesses a relatively high degree of reactivity and considerable attention has been devoted to the formation of various classes of compounds containing a plurality of aziridino groups. For example, the aziridino group may appear as a phosphoramide (U.S. Pat. No. 3,226,377), as an alkyl or alkylene aziridine (U.S. Pat. No. 3,231,563), as a substituent of an epoxy compound (U.S. Pat. No. 3,240,720), as a carbamate (U.S. Pat. No. 3,223,681) or as an aziridino substituted ether (U.S. Pat. No. 3,187,463). As indicated by the general structural formula given above, the present invention concerns aziridino compounds which contain a plurality of aziridino groups as substituents on a specific class of hydrocarbons.
The patents noted above disclose a variety of ways in which aziridino compounds may be prepared. It is contemplated that new compounds of this invention and other products which are useable in accordance with the invention in the curing of carboxyl containing polymers may be prepared in any suitable fashion known to the art. However, the preparation of hydrocarbon diaziridines from corresponding hydrocarbon dihalides by condensation with alkylenimines, either as the free alkylene imine or the corresponding alkali metal salts, in the presence of a scavenger, such as amines (see U.S. Pat. No. 3,197,463) may be advantageously used in producing multiaziridines for the invention. Some of the multiaziridines may also be produced by the addition of an alkylenimine, such as ethylenimine, to a vinyl aromatic compound, e.g., divinyl benzene, di-isopropenyl benzene, and the like, in presence of an alkali metal catalyst such as sodium, potassium, and the like.
Since diaziridines contain a plurality of reactive groups, they have been investigated for use as curing agents for various natural and synthetic polymeric materials. Their use in the modification of natural and synthetic fibers and in the tanning of hides and skins by recourse to their multifunctional groups has been previously disclosed, e.g., see U.S. Pat. No. 3,169,122. A variation of this concept is the use of diaziridines in the treating of fibers and textiles to crease-proof or dimensionally stabilize the textiles, e.g., see U.S. Pat. No. 3,165,375. In a broader sense, the general concept of using multiaziridino compounds as cross-linking agents for polymers containing active hydrogens or their equivalents to produce three-dimensional polymer networks, thereby providing solid materials of improved properties has been disclosed, e.g., see U.S. Pat. No. 3,162,618. The present invention concerns improvements in the field of multiaziridino compounds and in their use as curing agents for certain classes of polymers and the discovery that particular forms of multiaziridines when used as curing agents for carboxyl containing polymers provide improved characteristics in resulting cured products as compared with the curing of such polymers by procedures known heretofore. In this connection, polycarboxylic organic compounds have previously been cross-linked with polyaziridinyl phosphorous compounds to yield cross-linked resinous and rubber compositions. This procedure was found to involve serious problems because the phosphorous compounds employed were not stable and could not be obtained in a consistently pure form.